1. Field of the Invention
The present invention relates to an automatic gain control circuit and a method thereof, and particularly relates to an automatic video gain control circuit for positive and negative modulation and a method thereof.
2. Description of the Prior Art
A digital television system should be capable of decoding digital TV signals complying with standards such as ATSC (Advanced Television Systems Committee, ATSC) or SECAM (Sequential Couleur Avec Memory). Also, a digital television system should be capable of decoding TV signals complying with conventional analog TV system specifications such as NTSC (National Television Standards Committee) and PAL (Phase Alternation Line).
FIG. 1 shows a front-end module of a prior art video system, including a tuner 101 and a demodulator 103. The tuner 101 converts a video radio frequency signal RF to a video intermediate signal VIF, and the demodulator 103 converts the video intermediate signal VIF to a composite video signal CVBS. Normally, the video intermediate signal VIF is in a range 41-46 MHZ for an NTSC specification.
Conventionally, the demodulator 103 performs a sync white detection to control the gain of the radio frequency signal RF. Thus, all white frames are required for gain control. The demodulator 103 will be over-saturated if black frames continuously appear for a period of time. Particularly, a positive modulation or a negative modulation may be applied in a digital TV RF signal. The sync white detection cannot meet requirements for both kinds of modulation. As noted, the sync white detection is easily affected by the video content.
Therefore, an automatic video gain control circuit that will not be affected by video content is needed for various video signal specifications.